1. Field of the Invention
The present invention relates to sintered bodies of aluminum nitride used as materials for insulating substrates, heat sinks, packages for semiconductor devices, etc. More specifically, the present invention relates to sintered bodies of aluminum nitride having a high density and a high thermal conductivity which can be produced at reduced sintering temperatures.
2. Description of the Prior Art
In recent years, with an increasing trend toward the high integration and high densification of LSI, there has been a great demand for insulating substrate materials having a good thermal conductivity. Heretofore, sintered alumina has been most widely used as a material for an insulating substrate. However, since the alumina substrate has a low thermal conductivity (about 20 W/m.K) and has a large thermal expansion coefficient as compared to silicon, it has many problems such as poor bondability, etc. Therefore, sintered bodies of aluminum nitride (AlN) which have a good thermal conductivity and a thermal expansion coefficient approaching that of silicon have attracted a great interest. In addition to these advantages, the aluminum nitride sintered bodies possess a high mechanical strength (40-50 kg/mm.sup.2) and good electrical properties, for example, a small dielectric constant However, since aluminum nitride materials are poor in sinterability due to their strong covalent bond, it is difficult to obtain high density sintered bodies.
For this reason, various ways of achieving high density sintered bodies of aluminum nitride have been tried up to date and, for example, there may be mentioned a hot press sintering method and a normal pressure sintering method, both methods using a sintering aid. The hot press sintering method is unfavorable because of poor productivity. In the normal pressure method, sintering aids, such as compounds of rare earth or alkaline earth elements have been used and sintered bodies of aluminum nitride having a high density and a high thermal conductivity have been obtained. However, such a known method requires high sintering temperatures exceeding 1800 .degree. C. and is unfavorable from the viewpoint of productivity.
On the other hand, as means for improving productivity, Japanese patent application laid-open No. 61-117160 discloses a method for producing a low temperature sintered body of aluminum nitride in which rare earth oxide and alkaline earth oxide are added in combination. This method tends to cause uneveness in sintering and coloration and can not still provide satisfaction in thermal conductivity. Further, sintered bodies of aluminum nitride with a high thermal conductivity and a production method thereof are described in Japanese Patent Application laid-open Nos. 61 - 270 262 and 61 - 270 263, the sintered bodies consisting of boride or carbide of Groups 4a, 5a or 6a elements of the Periodic Table and aluminum nitride.
In such circumstances, further improvements have been needed in order to obtain higher thermal conductivity.
Previously, Inventors have found that addition of CaMoO.sub.4 and/or CaWO.sub.4 enables sintering at low temperature, as described in Japanese Patent Application No. 61 - 270 39, substantially corresponding to U.S. Ser. No. 118,192. However, the thermal most, on the order of about 100 W/m.K, as referred to in the other above Japanese Patent Application Laid-open Nos. 61-117 160, 61-270 262 and 61-270 263.
Also, as a method recently proposed, addition of TiO.sub.2 or ZrO.sub.2 in combination with Sc.sub.2 O.sub.3 is described in Japanese Patent Application Laid-open No. 62- 72 570. However, this method is economically unfavorable in view of the use of expensive Sc.sub.2 O.sub.3 and the thermal conductivity of the resulting sintered bodies is at most about 100 W/m.K.